API is Application Programming Interface. API's define a set of rules for programmers to follow when interacting a software. In simpler terms, API allows for one program to interact with another program. API's can be used in trading. API can assist traders who run algorithmic models to execute trades on a system.
ProBit Exchange supports the utilization of API on the trading platform.
You can find more details in an article found at this link (https://bravenewcoin.com/insights/what-is-api-trading-and-how-is-it-applied-to-crypto).